Prior art single transistor ferroelectric devices may be classified in two types of devices: Metal-Ferroelectric-Metal-Oxide Semiconductor (MFMOS) transistors and Metal-Ferroelectric-Metal-Semiconductor (MFMS) transistors. The structures of such devices are depicted in FIG. 1 and FIG. 2, respectively.
Referring initially to FIG. 1, a MFMOS memory transistor is depicted generally at 10. Transistor 10 is constructed on a silicon substrate 12. The transistor includes a gate region 14, a n+ source region 16, a n+ drain region 18 and a ferroelectric (FE) gate stack 20. Gate stack 20 includes a bottom electrode 22, a FE layer 24, and a top electrode 26. An oxide insulating layer 28 covers the conductive portions of the transistor. The completed transistor includes a source electrode 30, a gate stack electrode 32, and a drain electrode 34. As shown in FIG. 2, a MFMS memory transistor 36 is similarly constructed to transistor 10, but includes an n- layer 38 in gate region 14.
The materials used in the FE stack for the top and bottom electrode in known ferroelectric memory transistors are Pt, Ir, Zr, IrO, ZrO, or alloys containing one or more of the metals. To insure proper operation of the completed device, the gate stack has to be precisely etched to align the sides of the ferroelectric capacitor. Although equipment is available for performing such etching on the metals, the etch, is at best, a sputtering process, which is only partially successful. It is not possible to selectivity etch the metal without damaging the surrounding silicon oxide and silicon to a degree that is acceptable, which requires that gate stack plasma etching consumes the surrounding silicon and oxide in amounts less than several tens of nanometer. Any consumption greater than this amount will degrade or destroy the normal operation of the memory transistor.